1. Field of the Invention
The present invention relates to a multi-layered-type liquid-crystal display device which uses a twisted nematic display process that can produce a black-and-white display image and a colored display image with high contrast.
2. Description of the Prior Art
Liquid-crystal display devices are now being used in clocks electronic calculating machines, computer terminals, word processor displays, televisions, and in a variety of other display devices in many fields. Recently, there has been an extremely large demand for liquid-crystal display devices because of the changes to multicolor and full-color displays, which are already being used in the fields of graphic display and image display. These color displays use a liquid-crystal cell with color filter layers. The liquid-crystal cell functions as a light-switch and produces various colors. The main kind of display mode is a twisted nematic (TN) display mode attained by a liquid-crystal cell in which the liquid-crystal molecules are twisted 90.degree. so that high contrast, etc., can be obtained. However, by using this TN display mode, the dependence of the display characteristics on the wavelength of light is great, and it is not possible to achieve uniform switching of light over the entire spectrum of visible light. In particular, when the normally closed display process has the axes of absorbance of the two polarizers in parallel, there is the problem that light leakages at the time of the application of zero voltage gives rise to color.
There are two kinds of driving methods to drive a TN color display device that utilizes light-switching this kind of a color filter layer, there are two main. One of these methods involves the active-matrix drive method that uses a liquid-crystal cell having picture elements provided with non-linear devices such as diodes or switching elements such as thin-film transistors. The other method involves the multiplex drive method in which the liquid crystals of a liquid-crystal cell having no picture elements are successively driven. With the latter method, the steepness in the vicinity of the threshold value of the optical characteristics of the liquid crystals is important. This characteristic is a problem with the TN displays that are being used now. In order to improve the optical characteristics so that the proper steepness in the vicinity of the threshold value can be attained, there has been suggested a supertwisted birefringence effect (SBE) process that causes liquid-crystal molecules to be twisted at angles of around 180.degree.-270.degree.. With the SBE process, the curve in the vicinity of the threshold value increases sharply such that even when the duty ratio increases, it is possible to obtain a high contrast ratio. However, because birefringence effects of liquid crystals are used, the dependence of the display characteristics on the wavelength is theoretically higher than with the typical TN display, thereby making it very difficult for use in a full-color display.